1. Field of the Invention
The present invention relates to an induction sealing apparatus which seals a foil sheet or liner to the opening of a container and more particularly to an induction sealing apparatus which is air cooled.
2. Related Art
Induction sealing units for sealing, hermetically sealing or tamper-proof sealing a container with a foil liner are typically included in conveyor systems for high volume applications. These systems usually have flat or tunnel sealing heads mounted above a conveyor, which conveyor carries a plurality of containers to be sealed into proximity with the sealing head.
The containers to be sealed are preconditioned to include a foil liner disposed over the opening of the container. Usually, the foil liner is held in place by a screw-on or snap-on cap as is known in the art. Sometimes a wax compound and a paperboard portion are included above the foil liner.
Once the preconditioned container is brought within a predetermined distance from the sealing head, a coil within the sealing head produces an electromagnetic field near the foil liner which is disposed within the cap. The electromagnetic flux produced by the field causes current to flow in the foil liner which causes the foil liner to heat and melt the lip of the container (and wax compound if used). The molten plastic adheres the foil liner to the opening of the container and, when the plastic cools, a seal is produced. The downward force supplied by the cap ensures a proper bond between the foil liner and the opening of the container, particularly when the molten plastic is cooling.
Typically, power supplies which include solid state switching circuits generate high frequency currents for delivery to the coil within the sealing head to produce the requisite electromagnetic field for heating the foil liner. In addition, the coil in the sealing head is disposed around dense ferrite materials to channel the electromagnetic field and direct the field flux towards the foil liner for improved performance. Generally, the power supplies deliver approximately 2 to 4 kilowatts of power to the sealing head in order to properly seal the container.
In conventional systems, high currents are produced in the coil of the sealing head and, due to the relatively high resistance of the coil, results in a drastic elevation of the temperature of the coil and any surrounding structures. Therefore, the induction sealing units of the prior art employ hollow copper tubing when forming the coil in the sealing head to permit cooling water to flow through the hollow copper tube coil and cool the sealing head.
Unfortunately, the water cooling systems which cool the sealing heads (via hollow copper tubing coils) are expensive, bulky and inefficient. Such water cooling systems require tubing, radiators, radiator fans, circulators, pumps and the like in order to adequately cool the system.